Overview
This lecture covers polysaccharides, focusing on their structure, formation, and biological significance, particularly starch, glycogen, and cellulose.
Polysaccharide Basics
- "Polysaccharide" means many sugar units (monosaccharides) linked by glycosidic bonds.
- Monosaccharide: single sugar unit (e.g., glucose); Disaccharide: two monosaccharides joined (e.g., maltose).
- Polysaccharides typically refer to molecules with more than 20 sugar units.
Formation of Glycosidic Bonds
- Alpha glucose forms glycosidic bonds through condensation reactions (removal of water).
- Two alpha glucose molecules form maltose (a disaccharide); repeating this forms polysaccharides.
Starch as Energy Storage in Plants
- Starch is the main energy storage molecule in plant cells.
- Glucose is stored as starch to prevent diffusion out of cells and to avoid affecting water potential (starch is insoluble).
- Starch has two forms: amylose (unbranched, helical) and amylopectin (branched).
Amylose and Amylopectin Structure
- Amylose: made of alpha glucose with only alpha 1-4 glycosidic bonds, forming a helical structure.
- Amylopectin: made of alpha glucose with both alpha 1-4 and alpha 1-6 glycosidic bonds, resulting in a branched structure.
Glycogen in Animals
- Glycogen is the energy storage molecule in animal cells, structurally similar to amylopectin but more highly branched (more alpha 1-6 bonds).
- Glycogen prevents water influx by osmosis and cell bursting by being insoluble and non-diffusible.
Cellulose Structure and Function
- Cellulose is made of beta glucose with beta 1-4 glycosidic bonds, forming a linear structure.
- Every other beta glucose is rotated 180° to allow bond formation.
- Linear cellulose chains form hydrogen bonds, creating strong microfibrils and fibers for plant cell walls.
- Cellulose provides high tensile strength and rigidity to plant cells.
Key Terms & Definitions
- Monosaccharide — a single sugar molecule.
- Disaccharide — two monosaccharides joined by a glycosidic bond.
- Polysaccharide — many monosaccharides linked together.
- Glycosidic bond — covalent bond between sugar molecules.
- Condensation reaction — chemical reaction that joins molecules with the removal of water.
- Amylose — unbranched, helical starch made of alpha 1-4 bonds.
- Amylopectin — branched starch with alpha 1-4 and alpha 1-6 bonds.
- Glycogen — highly branched polysaccharide in animals.
- Cellulose — linear polysaccharide of beta glucose with strong hydrogen bonding forming fibers.
Action Items / Next Steps
- Study the structures and differences of amylose, amylopectin, glycogen, and cellulose.
- Prepare to learn more about hydrogen bonding in the next lesson.